Sharing of information is a very delicate subject. One might argue that you don't need to leave a company to share that information with someone. I think it's bit about personality and loyalty; or your morality.
In Coke case it wouldn't make any difference to workers to know the formula, but there are lots of situations, like the one Francisco told in his blog, really would affect the efficiency of the employee; how can a consultant work without knowing about the company anyway.
So, choose your employees wisely ( I know its very hard) and create the environment to make sure to get their loyalty.
Obsessing that your employees will give the important information away is bit like thinking people will punch you : ) they won't as long as you don't give them any reason; but there is always a chance that there are mentally instable people : P
ok it is a bit silly, but anyway, you got the idea...